Electric machine having continuously-variable magnetic characteristics and method of controlling the same

Abstract

An electric machine produces motor torque having continuously variable magnetic and reluctance torque components. The electric machine includes stator and rotor assemblies. Different ends of the rotor hub have different average magnetic field strengths, with the second field strength at one end being weaker than the other. The rotor hub translates along the rotor shaft to vary the magnetic and reluctance torque components at different speed and torque operating points of the electric machine. A vehicle includes the machine, a transmission, a load, and a controller executing a method for controlling the axial position of the rotor hub. The method may include determining the speed and a torque of the electric machine, determining a corresponding desired axial position of a rotor hub, and translating the rotor hub to the desired axial position.

The invention claimed is: 
     
       1. An electric machine operable for producing a total motor torque having continuously-variable magnetic and reluctance torque components, the electric machine comprising:
 a stator assembly having a stator core with an axial length; 
 a rotor assembly that includes a rotor shaft and a cylindrical rotor hub each having respective axial lengths, the cylindrical rotor hub having a first end with a first magnetic field strength and a second end with a second magnetic field strength, wherein the second magnetic field strength is weaker than the first magnetic field strength; and 
 a return spring attached to the rotor hub; 
 wherein the rotor hub is translatable with respect to the rotor shaft, the axial length of the rotor hub exceeds the axial length of the stator core, and the rotor hub is configured to translate in a first direction along the axial length of the rotor shaft, via an apply force from an actuator or via introduction of a flux-weakening stator current to the stator windings, to continuously vary a relative contribution of the magnetic and reluctance torque components to the total motor torque in response to different speed and torque operating points of the electric machine, and to translate in a second direction via operation of the return spring. 
 
     
     
       2. The electric machine of  claim 1 , wherein the axial length of the rotor hub is at least 150 percent of the axial length of the stator core. 
     
     
       3. The electric machine of  claim 2 , wherein the stator hub is wound with stator windings such that the stator assembly has a total axial length equal to or exceeding the axial length of the rotor hub. 
     
     
       4. The electric machine of  claim 1 , wherein the stator core is separated from the rotor hub by a radial air gap that is uniform along the axial length of the stator core. 
     
     
       5. The electric machine of  claim 1 , wherein the rotor hub includes a plurality of permanent magnets connected to or embedded within the rotor hub. 
     
     
       6. The electric machine of  claim 5 , wherein a number of the permanent magnets at the first end exceeds a number of the permanent magnets at the second end. 
     
     
       7. The electric machine of  claim 1 , wherein the rotor hub is mechanically uniform along the axial length of the rotor hub. 
     
     
       8. The electric machine of  claim 1 , wherein the torque operating points include a low-speed/high-torque operating point at which the magnetic torque component is about 75 percent and the reluctance torque component is about 25 percent, and a high-speed/low-torque operating point at which each of the magnetic torque component and the reluctance torque component is about 50 percent. 
     
     
       9. A vehicle comprising:
 a load; 
 a transmission connected to the load, and configured to provide an output torque to the load responsive to an input torque; 
 an electric machine operable for producing the input torque to the transmission, the input torque being a total motor torque having continuously-variable magnetic and reluctance torque components, the electric machine including: 
 a stator assembly having a stator core with an axial length; 
 a return spring attached to the rotor hub; and 
 a rotor assembly including a rotor shaft and a cylindrical rotor hub each having a respective axial length, the cylindrical rotor hub having a first end with a first magnetic field strength and a second end with a second magnetic field strength, wherein the second magnetic field strength is weaker than the first average magnetic field strength; 
 wherein the rotor hub is splined to the rotor shaft, the axial length of the rotor hub exceeds the axial length of the stator core, and the rotor hub is configured to translate along the axial length of the rotor shaft, via an apply force from an actuator or via introduction of a flux-weakening stator current to the stator windings, at different speed and torque operating points of the electric machine, and to translate in a second direction via operation of the return spring; and 
 a controller configured to receive a set of input signals, including a speed and a torque of the electric machine, and to command a translation of the rotor hub along the axial length of the rotor shaft responsive to the set of input signals to thereby continuously vary a relative contribution of the magnetic and reluctance torque component to the total motor torque. 
 
     
     
       10. The vehicle of  claim 9 , wherein the axial length of the rotor hub is at least 150 percent of the axial length of the stator core. 
     
     
       11. The vehicle of  claim 9 , wherein the rotor hub includes a plurality of permanent magnets connected to or embedded within the rotor hub. 
     
     
       12. The vehicle of  claim 9 , wherein the rotor hub is mechanically uniform along the axial length of the rotor hub. 
     
     
       13. The vehicle of  claim 9 , wherein the controller is configured to energize the actuator to thereby actively translate the rotor hub along the axial length of the rotor shaft. 
     
     
       14. A method for controlling an electric machine operable for producing a total motor torque that includes continuously-variable magnetic and reluctance torque components, the electric machine having a stator assembly and a rotor assembly, the method comprising:
 determining a speed and a torque of the electric machine via a controller and at least one sensor; 
 determining a desired axial position of a cylindrical rotor hub of the rotor assembly via the controller using a lookup table, the desired axial position corresponding to the received speed and torque, wherein the cylindrical rotor hub has a first end with a first magnetic field strength and a second end with a second magnetic field strength, wherein the second magnetic field strength is weaker than the first magnetic field strength; and 
 translating the rotor hub in a first direction along the axial length of a rotor shaft of the rotor assembly to the determined desired axial position such that a relative of the magnetic and reluctance torque components to the total motor torque is continuously varied in real time responsive to the determined speed and torque, including energizing an actuator connected to the rotor hub or introducing a flux-weakening stator current into stator windings of the stator assembly; and 
 translating rotor hub in a second direction via operation of a return spring connected to the rotor hub. 
 
     
     
       15. The method of  claim 14 , wherein translating the rotor hub along the axial length of the rotor shaft of the rotor assembly and to the desired axial position to thereby automatically vary the magnetic and reluctance torque components includes varying the magnetic torque component between about 75 percent and 50 percent at a low-speed/high-torque operating point of the electric machine, and varying the reluctance torque component between about 25 percent and 50 percent at a high-speed/low-torque operating point of the electric machine.